DOC: fix code-block in the reshaping docs

[jorisvandenbossche]

The code block at this location:

https://dev.pandas.io/docs/user_guide/reshaping.html#pivoting-with-single-aggregations

is not rendered correctly, as we accidentally try to render it as a note. The .. note:: should be replaced with .. code-block::

[espatatis]

Can I get this issue assigned ?

[jorisvandenbossche]

@espatatis sorry, there was already a PR opened by someone else: #28820